1. Field of the Invention
This invention relates to an auto tensioner for automatically holding a belt tension at an appropriate strength in a belt driving mechanism, and more particularly to an auto tensioner for suppressing a change of the position of a tension pulley due to a vibration or impact received from a belt as well as allowing a pressing work of the tension pulley so as to keep the belt tension constant.
2. Description of the Related Art
Some type of the auto tensioner comprises a tension pulley on which a belt is to be wound, an arm for supporting the tension pulley such that it is capable of rotating, a fixing shaft for supporting this arm swingably, a friction plate for supplying a swing resistance to this arm, and a twisting coil spring for urging the arm in the direction of a rotation thereof as well as pressing the arm against the friction plate. The twisting coil spring and the friction plate exert different functions between when the belt tension changes mildly and when a violent vibration or impact is applied thereto.
First, if the belt tension changes mildly, the arm is inclined by the twisting restoration force of the twisting coil spring, so as to keep the belt tension constant. If a violent vibration or impact is applied from the belt to the tension pulley, that vibration or impact is transmitted to the arm, however because the arm is pressed against a friction plate by the expansive restoration force of the twisting coil spring, a large friction resistance is generated between the arm and the friction plate. As a result, the vibration or impact is absorbed and damped quickly, thereby preventing the swing of the arm supporting the tension pulley so as to keep the belt tension constant.
In the above auto tensioner, the friction plate is worn with a passage of time. Then, wearing particles are generated by this wearing. Some part of the wearing particles remain in a gap between the friction plate and a mating member which the friction plate contacts. If the remainder of the wearing particles increases, the friction resistance between the friction plate and the arm changes. Consequently, the damping function for the vibration or impact may drop. As a result, there is a possibility that a function of the auto tensioner for protecting the arm supporting the tension pulley from the aforementioned vibration or impact so as to keep the belt tension constant may be lost.